How Venus Transit of Sun in June May Help Find Alien Planets

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Observations of next month's historic Venus transit may
eventually help astronomers spot and study alien planets circling
faraway stars, one prominent researcher says.

On June 5, Venus will cross the face of the sun from Earth's
perspective — the last time it will do so for 105 years. But the
upcoming
Venus transit of the sun is more than just a rare skywatching
treat; it's also a great opportunity to hone our techniques for
hunting down and characterizing alien planets.

"We're trying to do as much as we can to use the transit of Venus
to understand exoplanets and their atmospheres," Jay Pasachoff,
of Williams College, told SPACE.com.

Pasachoff wrote a commentary in this week's issue of the journal
Nature, which appeared online today (May 16), detailing the
research opportunities the Venus transit provides. [ The
2012 Venus Transit: Complete Coverage ]

Searching for exoplanets

Looking for transits is one of the most productive ways to find
alien
planets. NASA's Kepler space telescope, for example, has
detected roughly 2,300 exoplanet candidates using this method,
which flags the telltale dips in a star's brightness caused by a
transiting planet.

The vast majority of these potential planets still need to be
confirmed, but Kepler scientists estimate that at least 80
percent of them will end up being the real deal.

One potential issue with the transit method is that brightness
dips can be caused by a variety of factors other than
light-blocking planets. For instance, dark patches known as
starspots — akin to the sometimes
massive sunspots seen on our own sun — can reduce a star's
luminosity slightly.

Next month's
Venus transit comes during an active period in the sun's
11-year activity cycle, and it's likely that some sunspots will
darken the solar surface on June 5. So the transit could give
astronomers practice in picking up a planet's signal around a
spotty, variable star, Pasachoff said.

The last Venus transit, in 2004, didn't offer that opportunity,
since it occurred during a quiescent phase in the solar cycle
when the sun's face was largely spot-free. (Venus transits occur
in pairs eight years apart, but these dual events happen less
than once per century.) [ Venus
Transit of 2004: 51 Amazing Photos ]

Characterizing planets

Venus' trek across the solar disk may also eventually help
researchers better understand exoplanets and their atmospheres,
Pasachoff said.

For starters, careful study of the transit will allow astronomers
to calculate Venus' diameter, which is already known. By
comparing the two numbers, scientists may get a better idea of
how accurately this technique can be used to estimate exoplanet
sizes.

Further, scientists who train their instruments on the transit
can learn a great deal about the nature and composition of Venus'
thick atmosphere. Because researchers already know quite a bit
about Venus' air, the transit could serve as a sort of
calibration exercise for future exoplanet studies.

Pasachoff and his colleagues are planning to do some work of this
sort. To look for carbon dioxide, a major component of Venus'
atmosphere, they'll put a new filter over a massive spectrograph
at the National Solar Observatory in New Mexico.

"This will provide a unique, detailed spectrographic study of a
relatively well-known atmosphere during a transit, which we can
compare to studies of unknown
exoplanet atmospheres," Pasachoff writes in the Nature
commentary.

And it's not as if scientists know all there is to know about
Venus' air. Pasachoff is working with researchers around the
world to observe the transit with multiple instruments, in an
effort to learn more about the nature and evolution of Earth's
hellishly hot "sister planet."

"We can use this in liaison with the data coming from the
European Space Agency's Venus Express [spacecraft] to understand
an intermediate level of Venus' atmosphere better than we can
from the spacecraft alone, or from the transit alone," Pasachoff
told SPACE.com.

Pasachoff's commentary in Nature is essentially a call to action.
He's urging his colleagues to take full advantage of the Venus
transit, even if they're not exactly sure how useful their data
will end up being.

"It is too soon to know exactly how the study of transits in our
solar system will help us to interpret observations of distant
exoplanets, but transits are so rare that to squander these
opportunities would be a crime," Pasachoff writes.

"We owe it to future astronomers — especially those who will
observe the next transit of Venus, in 2117 — to collect as much
data as possible," he adds. "One never knows what will prove
vital to future research."

Some other astronomers apparently feel the same way. NASA's Solar
Dynamics Observatory spacecraft, for example, will watch the
seven-hour transit, partly to calibrate some of its instruments
and partly to learn more about Venus' atmosphere.

And NASA's Hubble Space Telescope will observe the transit too,
albeit indirectly. Because Hubble is too sensitive to be pointed
anywhere near the sun, it will
use the moon as a mirror, studying light that bounces off
Earth's nearest neighbor. The goal is to determine the makeup of
Venus' atmosphere, testing out a technique that astronomers could
use to study far-off exoplanets.

You can follow SPACE.com senior writer Mike Wall on
Twitter:@michaeldwall.
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